Oceanic plates experience extensive normal faulting as they bend and subduct, enabling fracturing of the crust and upper mantle. Debate remains about the relative importance of pre-existing faults, plate curvature and other factors in controlling the extent and style of bending-related faulting. The subduction zone off the Alaska Peninsula is an ideal place to investigate controls on bending-related faulting as the orientation of abyssal-hill fabric with respect to the trench and plate curvature vary along the margin. Here we characterize bending faulting between longitudes 161°W and 155ºW using newly collected multibeam bathymetry data. We also use a compilation of seismic reflection data to constrain patterns of sediment thickness on the incoming plate. Although sediment thickness increases by over 1 km from 156°W to 160°W, most sediments were deposited prior to the onset of bending faulting and thus have limited impact on the expression of bend-related fault strikes and throws in bathymetry data. Where magnetic anomalies trend subparallel to the trench (<30°) west of ~156ºW, bending faulting parallels magnetic anomalies, implying bending faulting reactivates pre-existing structures. Where magnetic anomalies are highly oblique (>30°) to the trench east of 156ºW, no bending faulting is observed. Summed fault throws increase to the west, including where pre-existing structure orientations do not vary between 157-161ºW, suggesting that the increase in slab curvature directly influences fault throws. However, the westward increase in summed fault throws is more abrupt than expected for changes in slab bending alone, suggesting potential feedbacks between pre-existing structures, slab dip, and faulting.

Seismic anisotropy produced by aligned olivine in oceanic lithosphere offers a window into mid-ocean ridge dynamics. Yet, interpreting anisotropy in the context of grain-scale deformation processes and strain observed in laboratory experiments and natural olivine samples has proven challenging due to incomplete seismological constraints and length scale differences spanning orders of magnitude. To bridge this observational gap, we estimate an in situ elastic tensor for oceanic lithosphere using co-located compressional- and shear-wavespeed anisotropy observations at the NoMelt experiment located on ~70 Ma seafloor. The elastic model for the upper 7 km of the mantle, NoMelt_SPani7, is characterized by a fast azimuth parallel to the fossil-spreading direction, consistent with corner-flow deformation fabric. We compare this model with a database of 123 petrofabrics from the literature to infer olivine crystallographic orientations and shear strain accumulated within the lithosphere. Direct comparison to olivine deformation experiments indicates strain accumulation of 250–400% in the shallow mantle. We find evidence for D-type olivine lattice-preferred orientation (LPO) with fast [100] parallel to the shear direction and girdled [010] and [001] crystallographic axes perpendicular to shear. D-type LPO implies similar amounts of slip on the (010)[100] and (001)[100] easy slip systems during mid-ocean ridge spreading; we hypothesize that grain-boundary sliding during dislocation creep relaxes strain compatibility, allowing D-type LPO to develop in the shallow lithosphere. Deformation dominated by dislocation-accommodated grain-boundary sliding (disGBS) has implications for in situ stress and grain size during mid-ocean ridge spreading and implies grain-size dependent deformation, in contrast to pure dislocation creep.

The breakup of supercontinent Pangea occurred 200 Ma forming the Eastern North American Margin (ENAM). Yet, the precise timing and mechanics of breakup and onset of seafloor spreading remain poorly constrained. We investigate the relic lithosphere offshore eastern North America using ambient-noise Rayleigh-wave phase velocity (12–32 s) and azimuthal anisotropy (17–32 s) at the ENAM Community Seismic Experiment (CSE). Incorporating previous constraints on crustal structure, we construct a shear velocity model for the crust and upper 60 km of the mantle beneath the ENAM-CSE. A low-velocity lid ( of 4.4–4.55 km/s) is revealed in the upper 15–20 km of the mantle that extends 200 km from the margin, terminating at the Blake Spur Magnetic Anomaly (BSMA). East of the BSMA, velocities are fast (4.6 km/s) and characteristic of typical oceanic mantle lithosphere. We interpret the low-velocity lid as stretched continental mantle lithosphere embedded with up to 15% retained gabbro. This implies that the BSMA marks successful breakup and onset of seafloor spreading 170 Ma, consistent with ENAM-CSE active-source studies that argue for breakup 25 Myr later than previously thought. We observe margin-parallel Rayleigh-wave azimuthal anisotropy (2–4% peak-to-peak) in the lithosphere that approximately correlates with absolute plate motion (APM) at the time of spreading. We hypothesize that lithosphere formed during ultra-slow seafloor spreading records APM-modified olivine fabric rather than spreading-parallel fabric typical of higher spreading rates. This work highlights the importance of present-day passive margins for improving understanding of the fundamental rift-to-drift transition.

Small-scale convection beneath the oceanic plates has been invoked to explain off-axis non-plume volcanism, departure from simple seafloor depth-age relationships, and intraplate gravity lineations. We deployed thirty broadband OBS stations on ~40 Ma seafloor in the equatorial Pacific, in a region notable for gravity anomalies measured by satellite altimetry elongated in the direction of plate motion. P-wave teleseismic tomography reveals alternating upper mantle velocity anomalies on the order of ±2%, oriented parallel to the gravity lineations. These features, which correspond to ~300-500 ˚K lateral temperature contrast, and possible hydrous or carbonatitic partial melt, are strongest between 150 and 260 km depth, indicating rapid vertical motions through a low-viscosity asthenospheric channel. Coherence and admittance analysis using new multibeam bathymetry soundings substantiates the presence of asthenospheric density variation, and forward modelling predicts gravity anomalies that qualitatively match observed lineations. This study provides observational support for small-scale convective rolls beneath the oceanic plates.